Titanium is an element with an atomic number of 22 in the periodic table. It is a subgroup element of the fourth period, that is, the IVB flag. In addition to titanium, this group of elements also includes zirconium and hafnium. Its common feature is that it has a high melting point and is on its surface at room temperature. A stable oxide film is formed....
Titanium is an element with an atomic number of 22 in the periodic table. It is a subgroup element of the fourth period, that is, the IVB flag. In addition to titanium, this group of elements also includes zirconium and hafnium. Its common feature is that it has a high melting point and is on its surface at room temperature. A stable oxide film is formed.
1. Ten characteristics of titanium
(1) Low density, high strength and high specific strength
The density of titanium is 4.51g/cm3, which is 57% of steel. Titanium is less than twice heavier than aluminum and three times stronger than aluminum. The specific strength (ratio of strength/density) of titanium alloy is the largest in common industrial alloys (see Table 2-1). The specific strength of titanium alloy is 3.5 times that of stainless steel; 1.3 times that of aluminum alloy; 1.7 times that of magnesium alloy, Therefore, it is an indispensable structural material for the aerospace industry.
Table 2-1 Comparison of density and specific strength between titanium and other metals
(2) Excellent corrosion resistance
The passivation of titanium depends on the presence of an oxide film, and its corrosion resistance in an oxidizing medium is much better than that in a reducing medium. High rates of corrosion occur in reducing media. Titanium is not corroded in some corrosive media, such as seawater, wet chlorine, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chlorides, sulfides and organic acids. However, in media that react with titanium to produce hydrogen (such as hydrochloric acid and sulfuric acid), titanium generally has a greater corrosion rate. However, if a small amount of oxidant is added to the acid, a passivation film will be formed on the titanium surface. Therefore, titanium is resistant to corrosion in strong sulfuric acid-nitric acid or hydrochloric acid-nitric acid mixtures, even in hydrochloric acid containing free chlorine. The protective oxide film of titanium is often formed when the metal encounters water, even in small amounts of water or water vapor. If titanium is exposed to a strong oxidizing environment with no water at all, it will oxidize rapidly and react violently, often even spontaneously igniting. Such phenomena have occurred in the reaction of titanium with fuming nitric acid containing excess nitrogen oxide and with dry chlorine. So to prevent such reactions, there must be a certain amount of water.
(3) Good heat resistance
Usually aluminum loses its original properties at 150°C, stainless steel loses its original properties at 310°C, and titanium alloys still maintain good mechanical properties at around 500°C. When the speed of the aircraft reaches 2.7 times the speed of sound, the surface temperature of the aircraft structure reaches 230 ℃, aluminum alloy and magnesium alloy can no longer be used, and titanium alloy can meet the requirements. Titanium has good heat resistance and is used in the disks and blades of aero-engine compressors and the skin of the rear fuselage of aircraft.
(4) Good low temperature performance
The strength of some titanium alloys (such as Ti-5AI-2.5SnELI) increases with the decrease of temperature, but the plasticity does not decrease much, and it still has good ductility and toughness at low temperature, which is suitable for use at ultra-low temperature. It can be used on dry liquid hydrogen and liquid oxygen rocket engines, or as ultra-low temperature containers and storage tanks on manned spacecraft.
(5) Non-magnetic
Titanium is non-magnetic, it is used in submarine hulls and will not cause the explosion of mines.
(6) Small thermal conductivity
The thermal conductivity comparison between titanium and other metals is shown in Table 2-2.
Table 2-2 Comparison of thermal conductivity between titanium and other metals
The thermal conductivity of titanium is small, only 1/5 of steel, 1/13 of aluminum, and 1/25 of copper. Poor thermal conductivity is a disadvantage of titanium, but this feature of titanium can be exploited in certain applications.
(7) Low elastic modulus
The elastic modulus comparison of titanium and other metals is shown in Table 2-3.
Table 2-3 Comparison of elastic modulus between titanium and other metals
The elastic modulus of titanium is only 55% of that of steel. When used as a structural material, the low elastic modulus is a disadvantage.
(8) Tensile strength and yield strength are very close
The tensile strength of Ti-6AI-4V titanium alloy is 960MPa and the yield strength is 892MPa, the difference between the two is only 58MPa, see Table 2-4.
Table 2-4 Comparison of tensile strength and yield strength between titanium and other metals
(9) Titanium is easily oxidized at high temperature
Titanium has a strong binding force with hydrogen and oxygen, so attention should be paid to preventing oxidation and hydrogen absorption. Titanium welding should be carried out under argon protection to prevent contamination. Titanium tubes and sheets should be heat treated under vacuum, and a micro-oxidizing atmosphere should be controlled during heat treatment of titanium forgings.
(10) Low anti-damping performance
The bells are made of titanium and other metal materials (copper, steel) with the exact same shape and size. If you strike each bell with the same force, you will find that the bell made of titanium oscillates for a long time, that is, through The energy given to the bell is not easily dissipated by the striking, so we say that the damping performance of titanium is low.
